The invention relates to multi-wavelength lasers, and more particularly, to a modulatable external cavity multi-emitter semiconductor laser with a predetermined wavelength spacing for DWDM applications.
Optical networks are becoming increasingly complex and use multiple wavelengths transmitted simultaneously over the same fiber. This transmission method is referred to as dense wavelength division multiplexing (DWDM), with the number of active channels continuing to increase. The international telecommunications union (ITU) standards body has proposed a channel allocation grid with 100 GHz channel spacing (xcx9c0.81 nm at a 1550 nm wavelength) on even 100 GHz intervals, counting nominally in both directions from a center frequency of 193.1 THz. Newer systems are being designed to reduce the channel spacing to 50 GHz or less. In addition, the total wavelength range over which these devices are designed to operate is increasing. Cost control is very important for system deployment. It would be very desirable to have one laser source module serve as the source for all the channels in the optical system. In addition, it is desirable to have the module be both scalable and upgradeable in a granular manner. Scaleable means that the system may be upgraded from small channel counts to larger channel counts. Granularity implies that the upgrades can be implemented one channel, or a few channels, at a time.
Optical communication systems are required to have a service life in excess of twenty years. Lasers should be easily reconfigurable and/or replaceable if one of the laser channels malfunctions. In addition, as channel count increases from the present channel count of less than 40 to greater than 100, it is very cost effective not to stock 100 different spares, but preferably at most a few xe2x80x9cgenericxe2x80x9d lasers that can be used for several wavelengths.
With a channel spacing of 25 GHz or less, wavelength stability of laser channel sources must be better than a few percent of the channel spacing. The component and system cost of new generations of networks that require greater stability and closer channel spacing should also not increase. Furthermore, crosstalk between channels should be less than 30 dB of the peak channel power which is typically in the range of 1-10 mW.
Devices that combine multiple wavelengths from different semiconductor laser sources have been reported. Integrated devices with an echelle grating or an arrayed waveguide grating as dispersive optical element have been built on a single semiconductor substrate. These devices require complex fabrication technologies and only allow limited, if any, wavelength tuning of the individual channels. In addition it is difficult to manufacture large channel count systems by these techniques, and replacement of individual channels is difficult.
Wavelength-tunable external cavity laser diodes have been employed in laser test equipment and provisioned as spares in optical networks. Multi-wavelength external cavity laser diode arrays with several emitters have also been reported, wherein the output is combined into overlapping beams that can be propagated wavelength-multiplexed, for example, through an optical fiber. However, such lasers, when electrically modulated, experience chirp which alters the output wavelength of the lasers, i.e. the carrier frequency, during the digital bit stream.
It would therefore be desirable to provide a laser source module that produces optical radiation for a plurality of channels in a specified band of an optical fiber transmission system, and more particularly a modulatable laser source module with improved wavelength stability.
The invention is directed to a multi-wavelength external-cavity laser system. An array of individual laser elements is placed in a shared laser cavity for all wavelengths defined by a free space grating and a single relay lens. Laser radiation from the cavity may be outputted as an array of wavelength-separated beams that can be individually modulated by external modulators or as an overlapping beam into a single fiber having substantially all the wavelengths. A modular design allows the addition and/or repair of individual channels or several channels.
According to one aspect of the invention, an external cavity laser source includes a free space external cavity and a plurality of optical modulators. The free space external cavity has a plurality of optical gain elements, wherein each of the optical gain elements generates optical radiation having a unique wavelength. The free space external cavity further includes a first optical element having a first focal plane, the first optical element positioned to substantially place the first focal plane at the plurality of optical gain elements and to intercept the generated optical radiation emitted from a first end face of optical gain elements; and a dispersive element positioned to intercept the radiation from the first optical element and diffracting the intercepted radiation. The diffracted radiation is returned through the first optical element to the first end face of the optical gain element that is associated with the unique wavelength.
This arrangement represents a shared cavity with separate optical gain regions.
Each of the plurality of optical modulators is associated with a respective one of the optical gain elements and adapted to intercept radiation from a second end face opposite the first end face of the respective optical gain element. Each of the optical modulators can therefore transmit a beam with a controllably modulated intensity at the unique wavelength.
The dispersive element can also operate in dual-pass which improves the overall wavelength resolution of the system. In this arrangement, the laser source includes a reflecting element, wherein the dispersive element is positioned between the first optical element and the reflecting element; and wherein the reflecting element is positioned to intercept radiation from the dispersive element and to retroreflect the intercepted radiation to the dispersive element.
In addition, the invention provides the ability to completely re-multiplex all channels into a single output fiber with or without requiring multi-fiber interconnection. This provides a very significant simplification in system design and a reduction in cost with significant functional improvements over standard distributed feedback lasers. According to this embodiment, a beam combiner intercepts the controllably modulated beams having the plurality of wavelengths and combines the controllably modulated beams into an overlapping beam. The beam combiner can include a second optical element having a second focal plane, with the second optical element positioned to substantially place the second focal plane at the plurality of the optical modulators and to intercept the controllably modulated beams from the optical modulators. A second dispersive element can intercept radiation from the second optical element and return the overlapping beam through the second optical element.
The dispersive elements can be free space gratings or immersion gratings, with the immersion gratings advantageously fabricated of silicon.
Further embodiments can include one or more of the following features. An optical receiving element, such as an optical fiber or waveguide or free space optics, can be placed in the second focal plane to receive the overlapping beam. Alternatively, the beam combiner can be an Nx1 optical coupler, such as a fiber coupler, a star coupler, or an arrayed-waveguide grating (AWG) coupler.
The external cavity can also include a spectral filter, for example, a Fabry-Perot etalon, which can be placed between the dispersive element and the reflective element. To stabilize the wavelengths, the laser source can further include a wavelength locker which derives an input signal from a unique wavelength of the plurality of wavelengths and controls the dispersive element and/or the spatial filter so as to collectively lock each wavelength of the plurality of wavelengths based on the input signal from the unique wavelength.
The extended length of the cavity allows very narrow laser line structure, with very high power in each channel. Furthermore, the invention enables all channels to be locked together to an external wavelength meter or molecular or atomic absorption cell referenced laser in order to economically prevent wavelength drift of all channels. Locking all channels together enables the potential to use coherent heterodyne detection schemes that can increase the system sensitivity by up to 20 dB. This should also provide even less expensive communications systems with the requirement of less signal restoration by Erbium Doped Fiber Amplifiers (EDFA) in long haul applications. It is also possible to substitute other gain media into the cavity such as optically pumped crystals and glasses.
Embodiments of the invention may incorporate one or more of the following features. Individual mirrors can be placed at the focus of the grating imaging optics operating as parallel output couplers, with the reflectivity of mirrors being tailored to the semiconductor lasersxe2x80x2 gain coefficients. The grating can be angle-tuned to different parts of the ITU grid.
The modulator array may be a lithium niobate modulator array, a planar waveguide electro-absorptive array, silicon Fabry-Perot etalon array or some other form of modulator known in the art.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments.